In recent years, transparent toothpastes have become well known oral cleaning products. At present, due to high chemical stability and good compatibility with various additives, silica is an essential raw material to produce the transparent toothpaste, and silica toothpaste is becoming more and more competitive in the market.
In order to make transparent toothpaste, it is necessary to make the refractive index of the liquid phase of toothpaste close to or even same as that of the solid phase. The liquid phase is composed mainly of water and humectant, sorbitol, polyethylene glycol and glycerin are commonly used humectants. In general, the refractive index of silica is in the range of 1.430-1.460, while the refractive index of 70% sorbitol is 1.457, the refractive indexes of water, polyethylene glycol, glycerol are 1.333, 1.46, and 1.47, respectively. The refractive index of a solution having a ratio of 70% sorbitol:water=90:10 is 1.4458, the refractive index of a solution having a ratio of 70% sorbitol:water=87.5:12.5 is 1.4390, the refractive index of a solution having a ratio of 70% sorbitol:water=85:15 is 1.4340, and the refractive index of a solution having a ratio of 70% sorbitol:water=80:20 is 1.4300. Through adjusting the ratio of humectant to water, it can make the refractive index of liquid phase coincide with that of the solid phase, thereby achieving the effect of transparency. In addition to the refractive index, the own transparency of the silica is also very important, generally the transparency of the silica requires more than 85%. As long as meeting the above two conditions at the same time, diffuse reflection will not occur, thereby better transparent toothpaste can be formed.
At present, the transparency of the silica with refractive index range of 1.440-1.460 can reach more than 90%, but the transparency of silica is lower than 80% in the low refractive index range of 1.430-1.440. In transparent toothpaste formulation, the water content is often low, normally about 10%, but the content of humectant such as sorbitol, polyethylene glycol or glycerin is about 65%, which leads to a high cost of the transparent toothpaste. Therefore, it is necessary to develop a new method to produce the silica with high transparency in the low refractive index, which can be used in a transparent toothpaste formulation to increase the content of water, while reducing the content of humectant, thereby reducing the production cost.